Manufacture of rolls



Nov. 3, 1942. H. A. LOMAX I MANUFACTURE OF ROLLS 2 Sheets-Sheet 1 Filed Nov. 10, 19,59

R H F. c o 0 3 o o O 4 2 O 3 3 a 4 5255 1 wwmznm r TEMPERATURE "F Nov. 3, 1942.

H, A. LOMAX MANUFACTURE OF ROLLS Filed Nov. 10, 1939 2 Sheets-Sheet 2 H9. 4-1600? F|- .5.-2ooo F.

Patented Nov. 3, 19 42 UNITED STATES PATENT 1 OFFICE MANUFACTURE or nous Harold A. Lomax, Wyomissinggla. Application November 10, 1939, Serial No. 303,835 7 Claims. (or. 148-215) This invention relates to rolls foruse in rolling metal, more particularly to alloy steel rolls, I

especially for use in rolling hot steel.

' A particularidisadvantage of both cast iron and alloy steel rolls made heretofore, is their tendency to fire crack or check. These surface. defects may be, and commonly are, of appreciable size, both as to width. and depth, and their occurence In accordance with the invention the alloy steel rolls are subjected to a heat treatment, or

annealing, in which they are heated to substantially above the critical temperature, most' suitably to at least about 1700 F., which is considerably above the critical temperature of the roll alloy steels. For many purposes a single heat necessitates resurfacing; by grinding or turning,

of the roll to remove them because if not removed they impair the surface finish of the rolled metal. The occurrence of such surface defects 'in the rolls thus increases rolling 'costs undesirably, not only due to the cost of grinding, but

also because of the reduction in roll diameter consequent upon removal of deep checks or frequent resurfacing.

A primary object of the invention is topro I vide a heat treatment for cast alloy steel rolls to confer resistance to fire cracking or checking superior to those of ordinary steel rolls. 1

Another object is to provide a heat treatment for cast alloy steel rolls' which confers superior resistance to fire cracking and checking and in the practice of which the hardness produced may be varied according to need as well as to produce rolls of greater" hardness than ordinarily developed heretofore in alloy steel rolls. v

It is also an object of this inventionto provide alloy steel rolls of simple and relatively inexpensive composition, which embody as a primary advantage high, resistance to checking and fire cracking that is produced by a simple heat treatment, and which possess suitable hardness together with other desirable physical and mechanical properties, such as strength and shock resistance, adequate for use in rolling metal, particularly in hot rolling of steel. Other objects of the invention from the following description.

The invention will be described with reference to the accompanying drawings in which Fig. 1 is a graph showing the hardness of a roll steel will appear alloy produced by cooling at different rates from ticular type of alloy steel.

treatment of this character suflices'both 'to con fer the improved resistance to fire cracking and checking and also to prepare the rolls for use.

But where they are subjected to" other heat treatments it is necessary either that this high temperature heat treatment be the final heating, or

that subsequent heat treatment be confined to heating belowthe critical range.

.Asis customary in these heat treating 'oper ations, it is usually desirable that the rolls be held at the high temperature for a period of time.

This heat treatment in accordance with the invention produces a new grain structure in the roll of a type which roll makers and users have sought heretofore to'avoid but which I believe isresponsible for, or contributes largely to, the superior resistance to fire cracking of rolls made in accordance with the invention. That is, when this heat treatment is the final step, or at least the final treatment above the critical tempera: ture, it causes recrystallization of the cast struc- 'ture with production of. grains surrounded by envelopes of cementite. The grains produced will be of increasing average size as the temperature of heat treatment increases, but in. general will be smaller than those of the original cast structure,: and as long as the temperature used is sufficiently above the critical range, they will possess the cementite envelopes referred to irrespective of the rate of cooling from temperature. This structure and the temperature necessary to produce it are illustrated in the drawings and discussed further hereinafter. Wherehigh hardness is desired the rolls are cooled at a relatively rapid rate from this heat treating temperature, and by variation of the heat treating temperature and the rate of cooling the hardness of the rolls may be varied within' wide limits. I r

By appropriate selection of composition, heat treating temperature and cooling rate, therefore,

there are produced rolls of'satisfacto'ry surface rolls. This improved resistance to fire cracking results from the application of the heat treatment provided by the invention to alloy steel rolls, particularly the nickel-molybdenum steels described hereinafter and which when heat treated as described herein provide particularly improved resistance to checking and fire cracking. More in detail, the surfaces of rolls made in accordance with the invention do not fire or water crack with production of either large horizontal or circumferential fire cracks such as have heretofore been customarily encountered in other alloy steel rolls, such checking or crazing as may occur being narrow and shallow, and there being no particular tendency for these fine cracks to enlarge substantially. In consequence, when it it is necessary to resurface the roll a perfectly satisfactory surface can be produced by removing as little as t( of an inch from the roll surface, so that the rolls are capable of repeated resurfacing with attendant long life.

The invention may be described further with reference to alloy steels which have been found to be particularly adapted to the practice of the invention in producing rolls of exceptional resistance to fire checking and cracking, viz., nickelmolybdenum alloy steels. Preferably the steels are of low, or relatively low, content of chromium. More particularly, the steels preferably used in the practice of the invention contain carbon from about 0.75 to about 2.5 per cent, chromium up to about 0.5 per cent, manganese up to about 1.5 per cent, molybdenum more than about 0.2 per cent and desirably at least about 0.35 per cent, and substantial amounts of nickel, the remainder of the steel being effectively iron together with elements and impurities, such as phosphorus, silicon and sulfur, in amounts customary in such alloy steels or which do not depreciate the desirable properties of the steels, i. e., the remainder is substantially all iron. Inasmuch as the rolls are made from steel alloys it ispreferred that the silicon content be relatively low, say not in excess of about 0.5 or 0.6 per cent. For many purposes it is preferred that the nickel content'of the steel be about three to four times the content of silicon, and ordinarily the nickel content will be from about 1.5 to 2.5 per cent. For many purposes not over about 0.75 per cent of molybdenum sufllces. Rolls made from steels of the foregoing com-. position and heat treated in accordance with the invention will possess a surface hardness of from 46 to 58 Shore scleroscope, or about 320 to about 410' Brinell, depending upon the particular composition within the ranges stated and upon the details of the heat treatment. By changes. in the,

ratio of the elements, for example nickel, silicon,

chromium and carbon, the hardness of the roll to cast the rolls from alloy steels containing from about 0.8 to 1.25 per cent of carbon, not over" about 0.5 per cent of chromium, from about 0.8 to

1.2 per cent of manganese, from about 0.3 to 0.5

the invention possess other physical properties which adapt them to use in rolling mills. For instance, they may possess a tensile strength of 110,000 pounds per square inch, so that they are extremely strong and of long life.

The particular temperature to which the rolls are heated and the precise rate at which they are cooled will depend, of course, upon the particular alloy steel composition used in making the roll, the temperature to-which it is heated, and the particular hardness or other mechanical properties desired. I have found by extended experience, however, that using an annealing temperature of 1700 F. coolingat an over-all rate of about F. per hour to well below the critical range, say to 800 F., is productive of eminently satisfactory results with rolls of the nickel-molybdenum steels described, and I believe this combination to be generally applicable to roll alloy steels to produce the results which characterize the invention. The rolls may, of course, be cooled more rapidly, say at 300 F. per hour, provided that this'does not cause them to be unsatisfactorily fragile. Heating at 1600 F. of these Ni-Mo steels does not produce a well-defined grain and cementite envelope structure, and cooling at the slow rates that have been used heretofore in annealing rolls, e. g., 25 F. per hour, does not produce the high hardness that is possible through the practice of the invention. I now believe that cooling rates of about 50 to about 300 F. per

hour suffice generally in the practice of the invention.

The effects of the annealing temperatures and cooling rates which characterize the invention may be exemplified further with reference to the drawings, which represent the results obtained a perature their hardness (ordinates in Fig. 1) was determined.

Reference to Fig. 1 shows that when the steel is cooled at the rate of about 100 F. per hour from 1600 F. the steel exhibits a Brinell hardness of about 285. However, with an annealing temperature of 1700" F. and the same cooling rate the hardness was 321, which confers more satisfactory wear resistance. As shown also by Fig, 1, the same hardness maybe had by cooling at 300 F. per hour from a lower temperature, 1. e., about 1550' F.

The graphs show further that for a given rapid cooling rate the hardness increases with the'annealing temperature, and that an increase in cooling rate results either in the production of equivalent hardness from a lower annealing temperature, or increase in hardness for a given temperature. Finally, samples cooled at 25 F. per hour developed maximum hardness when cooled from 1700 F., and their hardness was much less than is attainable with more rapid cooling rates.

Figs. 2 to 5 are photomicrographs at 100 diameters magnification of some of the samples cooled at 100 F. per hour used in obtaining the data for Fig, 1. As appears therefrom, in this series of tests the original cast structure persists up to an annealing temperature of 1600 F. (Fig. 2). At 1700 F. (Fig. 3), however, the structure has undergone recrystallization with production of grains that are smaller than in the original casting and'are more or less completely enveloped at their boundaries by cementite. This is 'truealsoof annealing temperatures above 1700 F. (Figs.

4-5) although as the temperature increases the grain size increases also.

This may explain thefsubstantially improved resistance to checking and fire cracking of rolls.

made in accordance with the invention. Thus, it

will be noted that in the structures produced at and above 1700 F. the hard cementite at the grain boundarywill tend to prevent transcrystallinerupture' in any grain frompassing' directly into an adjacent grain. Hence such a struclargement of checks formed at the. surfaceof the ture would tend to restrict the size and theenroll. On the other hand; it is reasonable to suppose that structures such as shown in Fig. 2 will not accomplish that result because once rupturestarts at the surface it is relatively freeto proceed unhampered and in a morev or lessstraight line through the large and relatively soft-grains.

to produce checks and cracks of large sizeand depth.

Although this presents a reasonable explana-.- 'tion' of the resultsbbtained in the practice of the invention, I do not limit myself to it because;-

7 it is not-known certainly whether the results are I claim: 1. As a new article of manufacture, a roll formed from cast alloy steel, saidroll being in heat treated condition, and having a recrystallized grain structure composed of grains smaller temperature below the critical range at a rate between about 25 and 300 F. per hour'and 'thereby'producing a structure of relatively coarse grains surrounded by cementite envelopes and heated to at least aboutl'ZOOv F. and being cooled due primarily to thestructural change"iust ..ex-.

plained, or whether it isdue primarily to the hardness developed by the heat ,treatment', lcoupled with other mechanical properties',--or per-f haps are .due bothto the. structure andi high hardness.- For example, rolls cooledat 300.

sistance to checking of rolls cooled at'1009 E.

per hour from 1700 F.-although the grain struc-" ture of-the formeris not so well developedas in the latter. However, it a fact that prior practice of cooling'slowlyfrom lower'temperatures, say at F. per hour from 1600 F.,'is not productive of the resistance to fire cracking which characterizes this invention. And the data given indicate the desirability of annealing at a temperature at which recrystallization occurs, and

the beneficial effects ofrapid cooling. As an example, a 29 x 69 inch blooming mill roll in accordance with the invention was heated to about 1700" F. in abouttwenty-eight hours and held at that temperature thirty hours, i. e., approximately, one hour for each inch of roll diameter, which is satisfactory in most cases. It was then cooled to about 800 F. in ten hours.

Although iii-general the desirable'practice is to subject the cast roll to a single annea1 as described, tests indicate that it is not necessary to restrict the heat treatment in this manner, i e., the essential thing seems tobe that as a final step the roll be cooled from an elevated temperature as described, or, alternatively, that after being thus heated any further heat treatment be below the critical range. According to the provisions of the patent statutes, I have explained the principle and mode of practicing my invention and have illustrated and described .what I now consider to represent its best embodiment. However, I desire to have it understood that, within th scope of the ap-- pended claims, the in vention may be practiced otherwise than as specifically illustrated and described.

rendering the roll resistant to fire cracking and checking. I

3. A method according to claim 2, the roll being at a rate of between hour. a 4. A method according to claim 2, the roll being made from an'alloycontaining about 0.75 to 2.5

about 50 and 300 F. per

per centofcarbon, from a small amount to not over about 0.5. percent'ofchromium, from a small .ar'nount-to not-over about 1.5 per cent of manganese, from about 0.2 to 0.75 per cent of molybdenum, from a small. amountto not over Q about 0.6 percent offsilicon, nickel from about .I three to-four times-the content of silicon, and

per hour fr0m"1600-F. appear to'possess-th'e re about 0.6 per cent of silicon, from about 1.5 to 2.5

the remainder substantially all iron.

-. Amethod according toclaim 2, the roll being made from an alloy containing about 0.75 to. 2.5

percent of carbon; from a small amount to not over about 0.5 per cent of chromium, from a small amount to not over about 1.5 per cent of manganese, from about.0.2 to 0.75 per cent of molybdenum, from a small amount to not over percent of nickel, and the remainder substantially all iron, and the roll being heated to at least about 1700 F. and being cooled at a rate between about and 300 F. per hour.

6. A method according to claim 2, in which the roll is made from an alloy containing about 0.8

'to, 1.25' per cent of carbon, from a small amount to not over about 0.5 per cent of chromium, from about 0.8 to 1.2 per cent of manganese, from about 0.3 to 0.5 per cent of molybdenum, about 1.6 to 1.8 per cent of nickel, from a small amount 7. As a new article of manufacture,.a rolling mill roll formed from east alloy steel containing about 0.75 to 2.5 per cent of carbon, from a small amount to not over about 0.5 per cent of chromium, from a small amount to not over about 1.5 'per cent of manganese, about 0.2 to 0.75 per cent of molybdenum, about 1.5 to 2.5 per cent of nickel, from a small amount to 0.6 per cent of silicon, and the remainder substantially all iron, said roll having a structure composed of recrystallized grains that are surrounded by cementite envelopes and are relatively coarse but smaller than the grains of the roll as cast, and the roll being highly resistant to checking and fire cracking.

HAROLD A. LoMAx. 

